This application is based on and claims priority to Japanese Patent Application No. 2000-379567, filed Dec. 14, 2000, Japanese Patent Application No. 2000-0379569, filed Dec. 14, 2000, and Japanese Patent Application No. 2001-052498, filed Feb. 27, 2001, the entire contents of which are hereby expressly incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, relates to an improved semiconductor device for power control in which a semiconductor chip is mounted on a substrate.
2. Description of Related Art
Semiconductor devices are utilized in wide variety of technical areas. For instance, a motor controller for an electrically operated vehicle such as, for example, an electric golf cart, includes a semiconductor device for power control of a motor that drives the golf cart. Typically, the semiconductor device that controls electric power is provided with one or more semiconductor chips that allow a relatively large current to flow therethrough. Because of the nature of power controlling semiconductor chips, the device can build much heat therein and needs a heat radiation structure. Thus, the semiconductor chips normally are placed on metallic heat spreaders that are mounted on metallic lands, which are formed on a metallic substrate, to expedite radiation of the heat. The heat spreaders are soldered onto the lands. The semiconductor chips are soldered onto the heat spreaders. Meanwhile, since a vehicle (for example, a golf cart) typically is used outdoors, the semiconductor device is exposed to a wet or dusty environment that can cause malfunctions of the device. In order to prevent the malfunctions from occurring, synthetic resin can be employed to entirely cover the semiconductor chips together with the lands.
FIGS. 1 and 2 schematically illustrate typical structures of a semiconductor device 10. A base metal 20 is coated with dielectric or insulating 22 to form a metallic substrate 24. Patterns of metallic lands 26 are formed on an upper surface of the substrate 24. Semiconductor chips 28 are joined to the metallic lands 26 via heat spreaders 30. More specifically, the semiconductor chips 28 are soldered onto the heat spreaders 30 with a solder layer 32 that has a relatively high melting point, and the heat spreaders 30 are then soldered onto the lands 26 with another solder layer 34 that has a relatively low melting point. Bonding wire pads 36 also are formed on the substrate 24 and are connected with the respective semiconductor chips 28 through bonding wires 38. After completion of the soldering and bonding processes, a synthetic resin 40 covers all the elements on the substrate 24.
FIG. 1 shows an example of the semiconductor device 10 that uses silicone gel for the synthetic resin 40. Because the viscosity of the silicone gel is relatively small, side walls 42 are applied to prevent the silicone gel from spilling before hardening. FIG. 2 shows another example of the semiconductor device 10 that uses epoxide. No side walls are necessary in this example because the epoxide has sufficient viscosity to stay on the substrate 24.
The heat spreaders 30 not only expedite radiation of the heat accumulating in the semiconductor chips 28 but also relieve the heat stress caused by disparity between the respective coefficients of the semiconductor chips 28 and the substrate 24. That is, the heat spreaders 30 cause the difference between the thermal expansion (or contraction) magnitude of the semiconductor chips 28 and the substrate 24 to be small. Accordingly, cracks of the solder layers 32, 34 or the semiconductor chips 28 and peeling of the semiconductor chips 28 from the solder layers 32, 34 are effectively prevented.
The usage of the heat spreaders 30, however, increases the number of parts, makes the semiconductor devices complicated and bulky, and increases the number of manufacturing processes. In addition, because the heat spreaders 30 are larger than the semiconductor chips 28, the lands 26 are inevitably large and require relatively wide spaces for them. Thus, the packaging density of the semiconductor chips 28 on the substrate 24 is diminished. A large casing that occupies a large area and has a large capacity may be necessary to accommodate the semiconductor chip 28.
A need therefore exists for an improved semiconductor device that has sufficient packaging density to make the device compact enough.
As thus far described, the soldering process is necessary for fixing the semiconductor chips onto the substrate. Several tools can be applied to hold the semiconductor chips in accurate positions, or each semiconductor chip can be soldered one by one for the same purpose. Both methods, however, need a number of steps and increase production cost accordingly.
Another need thus exists for an improved semiconductor device that can hold high accuracy of positioning of semiconductor chips without requiring expensive production cost.
In accordance with one aspect of the present invention, a semiconductor device comprises a substrate. A land is formed on the substrate. A semiconductor chip is mounted on the land. A solder layer is provided only through which the semiconductor chip is joined with the land. A synthetic resin covers the land, the solder layer and the semiconductor chip on the substrate.
In accordance with another aspect of the present invention, a semiconductor device comprises a substrate. A land is formed on the substrate. A semiconductor chip is mounted on the land. A solder layer joins the semiconductor chip with the land. The semiconductor chip defines at least two corners positioned generally opposite to each other. The land defines at least two corners disposed in proximity to the corners of the semiconductor chip. The corners of the land generally confine the corners of the semiconductor chip therein.
In accordance with a further aspect of the present invention, a method for joining a semiconductor chip to a substrate comprises forming a land on the substrate, soldering the semiconductor chip directly to the land, and covering the land and the semiconductor chip with a synthetic resin.